1. Field of Invention
The present invention relates to catalytic conversion of exhaust gases for internal combustion engines, and more particularly of heating a catalytic converter using cyclic fuel control and to detecting deterioration of efficiency of catalysts.
2. The Related Art
Catalysts for engine exhausts are used to convert unburned or partially reacted gases that are mostly made up of hydrocarbon (HC), carbon monoxide (CO), and nitrogen oxides (NOx) components. Gases leaving the exhaust manifold of an internal combustion engine enter the inlet of a device called a catalytic converter. A catalytic converter is the device that provides an expansive area where these gas components are oxidized to carbon dioxide (CO2), nitrogen (N2), and water vapor (H2O) by catalyst materials such as platinum (Pt), palladium (Pd), and rhodium (Rh). The conversion of these gases to CO2 N2 and H2O results in an exothermic chemical reaction at the catalyst surface that causes an increase in the temperature of the gases leaving the outlet side of the catalyst element. Increasing the concentrations of unburned or partially reacted gases entering the catalyst will result in a temperature rise that can be used to determine the catalyst""s conversion efficiency as well as to heat it. A catalytic converter can be made up of several catalyst bed elements (CE) that provide a large effective surface area for the catalyst material. Each catalyst element may have different oxidation characteristics for the gases entering from the engine""s exhaust manifold.
Other methods for determining the catalyst""s conversion efficiency by monitoring the level of the chemical or exothermic reactions occurring within a catalytic converter have been proposed in the past. Detection of the catalyst""s efficiency with a chemical method usually incorporates the use of two oxygen sensors placed at the inlet and the outlet portions of the catalytic converter catalyst elements being monitored. The voltage versus time characteristics of the two oxygen sensor""s output signals provides an indication of the catalyst""s ability for storing oxygen for chemical oxidation reactions. The catalyst""s oxygen storage capacity causes a difference between the catalyst""s inlet and outlet oxygen sensor signals. Since the catalyst""s oxygen storage capacity decreases after extended high temperature operation, the sensors output voltage versus time characteristics become more similar as the catalyst""s ability to store oxygen drops to zero.
However, the method using two oxygen sensors suffers from at least two problems. First, the accuracy of the oxygen sensor deteriorates over time thus creating sources of errors in detecting the actual deterioration in catalyst capability. For example, chemical factors such as fuel additives or sulfur concentrations can adversely affect the dual oxygen sensor method of catalyst efficiency monitoring. Second, the oxygen sensor method is dependent on the amount of active ceria in the catalyst rather than the catalyst""s oxidation conversion efficiency that is dependent on the other active precious metals. This results in a highly nonlinear relationship between catalyst efficiency and oxygen storage capacity that decreases the accuracy of catalyst efficiency monitoring. Catalyst efficiency durability characteristics sometimes are compromised to improve the correlation with the oxygen storage capacity and allow adequate catalyst monitoring accuracy.
Exothermic energy is also released at the catalyst""s surface during the chemical oxidation of hydrocarbons, carbon monoxide and nitrogen oxides (NOx) gases as they are converted into water vapor and carbon dioxide. The exothermic energy released in the catalyst causes a temperature increase at the surface of the catalyst""s substrate and in the exhaust gases flowing past this surface. Monitoring this temperature increase, caused by the exothermic energy release at the catalyst""s surface, provides a second method for measuring the catalytic converter""s overall chemical conversion performance.
Two primary methods have been proposed that monitor the catalyst""s gas conversion capability by using the catalyst""s temperature characteristics for determining its level of exothermic energy release. The first method uses two or more temperature sensors to monitor the temperatures of both the gases entering and exiting the desired portion of catalyst elements of the catalytic converters to be monitored. While flowing through the catalytic converter, each catalyst element""s outlet gas temperatures increase to a steady state level above its inlet exhaust gas temperatures depending on the exothermic energy released at the catalyst""s surface. Under some relatively steady state vehicle operating conditions, the temperature differences between the gases entering and exiting the portion of the catalytic converter being monitored provides an indication of the catalyst""s condition. U.S. Pat. Nos. 5,592,815 and 5,630,315 apply this first method of catalyst temperature detection during periods of steady state engine operating conditions. The steady state temperature difference between the inlet and outlet catalyst temperature sensors in a properly functioning catalytic converter can be about 50-80xc2x0 C. This compares with a catalyst having insufficient conversion efficiency with an indicated temperature sensor difference (outlet minus inlet) of 10-40xc2x0 C.
The second method of temperature based catalyst monitoring uses a momentary disabling of the ignition system voltage to the engine""s spark plugs to cause an unburned fuel and air mixture to exit the engine""s exhaust. The time period of disabling the ignition system must be short in order to prevent the torque change from the engine to be noticed by the vehicle""s driver. This pulse of unburned fuel and air mixture subsequently enters the catalyst and causes a sudden, momentary temperature rise of the catalyst""s temperature for a short time period. Quick responding temperature sensors are required to monitor this sudden and brief temperature rise at various portions within the catalyst where the unburned fuel and air mixture are oxidized. Temperature sensors must also be placed at the proper location where the unburned exhaust pulse will be oxidized since most catalytic converters have multiple catalyst elements with differing precious metal catalyst makeup. The location where the pulse will be oxidized is dependent on the instantaneous temperatures of each catalyst element. Identification of the instantaneous catalyst element temperatures and the location where the unburned exhaust pulse is oxidized can require multiple temperature sensors to be placed at various locations within the catalytic converter. This second method and related systems are shown in, for example, U.S. Pat. Nos. 5,339,628, 5,435,172, 5,355,671 and 5,610,844.
These two methods are dependent upon the ability of temperature detection devices or temperature sensors to accurately detect small temperature differences with magnitudes between 10-50xc2x0 C. Monitoring of the catalyst""s condition is performed during short time periods between 5-30 seconds when engine speed and load conditions are relatively stable. Normal temperature fluctuations caused by exhaust gases entering the catalyst during the catalyst efficiency monitoring time period are difficult to be accurately discerned from temperature changes caused by the catalyst""s exothermic reactions. Multiple temperature sensors are sometimes required with these methods to more accurately discern only temperature changes associated with the test for catalytic efficiency and its related exothermic chemical activity.
In the first method of monitoring, both the catalyst""s inlet and outlet temperature sensor""s error characteristics must remain very stable over the life of the engine in order to provide adequate detection of catalyst performance. This stability is required in the sensor""s output because a difference of only 10-40xc2x0 C. between the temperatures of the catalyst""s inlet and outlet gases can indicate the difference between a good versus failed catalyst. Conventional temperature sensors output errors can increase by more than 10xc2x0 C. during the catalyst""s useful lifetime. Also, the system to which the temperature sensor is connected can introduce additional measurement errors of a few degrees and further aggravate the accuracy of catalyst detection methods which depend on temperature differences below 40xc2x0 C. Therefore, small changes in the temperature sensor""s error characteristics during the catalyst""s lifetime and systematic error margins could cause an incorrect indication of the catalyst""s efficiency and result in premature replacement of the catalytic converter.
In the second method of catalyst pulse temperature monitoring, the long term accuracy stability of the temperature sensor is less critical since the detection method uses a temperature difference over a period of a few seconds. During this short time period, there""s no perceptible change in the sensor""s absolute output temperature reading. However, the second catalyst monitoring method requires the sensor""s response speed to closely follow the momentary catalyst gas temperature changes that occur after a pulse of unburned fuel and air enter the catalyst. This requires the temperature sensor""s time response characteristics to quickly follow the momentary temperature changes caused by the unburned exhaust pulse being oxidized as it passes through the catalyst. Changes in the sensor""s response time characteristics during the catalyst""s useful lifetime will affect the measured values of temperature change versus time characteristics required in the second method. The sensor""s response time characteristics must therefore be very stable over the sensor""s and catalyst""s useful lifetime for the second catalyst monitoring method to be practical. Variations in a sensor""s response time characteristics are caused by factors such as gas flow rates, catalyst radiant temperatures and aging. These factors are difficult to compensate for and add potential error sources when the period of catalyst efficiency testing evaluation is below 20 seconds.
Thus, these two methods require, that the temperature sensors accuracy to be very stable or response time characteristics to be very fast and stable over the catalyst""s and engine""s useful lifetime. Both of these requirements place high demands on temperature sensors that must operate at conditions of frequent temperature cycling and extremes exceeding 1000xc2x0 C. These requirements can significantly increase the cost of the sensor in order to meet both the performance and life expectancies when applying these two prior catalyst monitoring methods.
Further, each of the catalyst temperature monitoring methods are affected by many factors associated with normal engine operation. Temperatures of the inlet gases entering the catalytic converter can fluctuate due to the varying heat content of the engine""s exhaust gases at different speed (revolutions per minute or RPM), load conditions (torque) and other factors occurring in normal engine operation. Some transient engine operating conditions can result in large momentary variations in the air and fuel mixture entering the catalyst and these can affect the catalyst""s temperatures during a brief time period. These transient conditions could therefore introduce large potential errors in the second catalyst temperature monitoring method.
One purpose of this invention is to provide a method of consistently heating a monitored portion of a catalytic converter to cause an increase in the catalyst element""s temperature that can be accurately measured. Monitoring the exothermic characteristics of the catalyst element is not dependent upon chemical factors such as fuel additives or sulfur concentrations that can affect the dual oxygen sensor method of catalyst efficiency monitoring.
One objective of the present invention is to provide a system and method for controlling the engine""s fuel flow to provide a significant change in the catalyst""s operating conditions over a long time period that will provide conditions for inducing large, exothermic temperature changes that can be consistently monitored using durable, economical and practical temperature sensors.
Another objective of this invention is to provide a system and method for controlling the engine""s fuel flow to increase the rate of a catalyst""s heating shortly after the engine is started (engine startup), when temperatures are below 200xc2x0 C. in the catalyst, preferably without the use of an auxiliary air pump. It is advantageous to be able to quickly heat the catalyst after a cold start with the use of no auxiliary air pump or by using a smaller flow rate air pump to improve the system""s cost and reliability. An electrically or belt driven auxiliary air pump is sometimes used on engines to inject air directly into the exhaust system, at locations such as the exhaust manifold or into the catalytic converters, for temporarily providing excess oxygen into the engine""s exhaust system. An increasing number of engines will use an auxiliary air pump to help oxidize CO or hydrogen based fuel molecules at the catalyst elements"" surfaces, so that tail pipe emissions can be significantly reduced during cold engine start and warm-up, because of future emission regulations. Catalyst heating can be also be accelerated by controlling the engine to have a richer air-fuel mixture and then adding excess air with an air pump to produce exothermic reactions at the surfaces of the catalyst. Controlling the fuel rates to the engine""s individual fuel injectors, to provide excess oxygen into the catalyst, is an alternative method to reduce or eliminate the requirement of an air pump.
Cycling the catalyst""s inlet exhaust gas air-fuel ratio between rich and lean, by controlling the fuel quantities delivered to the fuel injectors 15 for each engine cylinder or groups of cylinders, provides an increased source of chemical energy to react at the catalyst elements"" surfaces. The catalyst""s inlet gas operating conditions are changed for a short time period following the initiation of exothermic catalytic activity using a cyclic fuel control heating method similar to that being proposed herein for use in diagnosing catalyst efficiency. The cyclic fuel control method proposed herein provides for varying the quantities of fuel delivered to the engine""s cylinders in a manner which produces alternating rich and lean cylinder exhaust gas air-fuel ratio characteristics that feed into the desired catalyst element. This system and method of heating a catalyst will provide increased level of chemical energy entering the catalyst that may combine exothermically for heating the catalytic converter to an efficient catalyst temperature quickly after initial engine startup and for diagnosing the efficiency of the catalyst.
Availability of cost effective temperature sensors with either, or both, long term accuracy stability and quick response to changes in the measured gas temperatures are the main challenges associated with prior art of catalyst efficiency detection employing temperature monitoring. A description of practical temperature sensors that can operate in the extreme high temperature environments within an engine""s exhaust system can be found in SAE technical paper number 942054 xe2x80x9cHigh Temperature Measurements for On-Board Diagnostics of LEV/ULEV Systemsxe2x80x9d by T. Tamai et al. These include temperature sensing devices such as thermocouples, thermistors and platinum resistive temperature devices (RTD) with established performance capabilities. Temperature sensors, with output characteristics that can reach 64% of the total measured gas step temperature change within a period of 5-10 seconds following the occurrence of the input gas temperature change, are considered to be quickest available designs. Quick responding and durable exhaust temperature sensors are typically referred to as having a response time of between 5-10 seconds.
In the present invention, a standard, high temperature, temperature sensor is placed in close proximity to the gas exit for only one of the catalyst bed elements in a catalytic converter that is selected to be monitored for determining the overall deterioration of the catalyst""s performance. The length, location and volume of the catalyst element to be monitored are together selected to provide a correlation between the overall catalytic converter""s gas conversion (chemical oxidation) efficiency and its temperature profile characteristics following a step change in exhaust gas levels of CO, HC and air. The temperature increase of the gases, caused by the exothermic oxidation reactions in an active catalytic converter, is dependent on the mass of unburned gases entering and the conversion efficiency of the respective catalyst material(s) for each gas. Accordingly, a catalyst is heated and a degradation in the catalyst""s conversion efficiency is detected by changing the concentration of CO, HC and oxygen in the engine exhaust gases entering an appropriately designed catalytic converter element and monitoring the temperature increase of the gases exiting this catalyst element. The temperature increase is provided in the present invention by cycling the fuel rate to the engine""s cylinders to cause a sufficiently large fluctuation in the air-fuel ratios of the gases entering in to the catalytic converter over a reasonable period of time during which the standard temperature sensor is able to detect a significant temperature change. The cycled cylinder groups are controlled in such a manner that the aggregate gas mixture is controlled to be, for example, at or near stoichiometric conditions and thus provide the necessary temperature increase while maintaining smooth engine rotation and minimizing tail pipe emissions. Therefore, the present invention is capable of providing a method of heating and monitoring the catalytic converter efficiency without adversely affecting the smooth engine rotation performance or fuel economy expected by a motor vehicle operator.
Further, with the present invention, the time period selected to cycle the exhaust gas air-fuel conditions to cause an elevated catalyst operating temperature may be controlled for a period of 50 seconds, or longer so the temperature sensor""s response time characteristics are not significant sources of measurement errors. The minimum time duration necessary is dependent upon engine and vehicle exhaust system design characteristics. Using an extended time period for operating the catalyst at elevated temperatures reduces sources of measurement errors associated with variations in catalyst aging and temperature sensor response time characteristic. The time period for using cyclic fuel is controlled by the engine control unit""s (ECU""s) 7 based upon values stored in its electronic memory and input control system parameters such as engine speed, engine load and catalyst element temperature for each specific engine and catalyst configuration. This provides the ability to accurately monitor the catalyst""s conversion efficiency while minimizing the total emissions from a vehicle""s tail pipe. A calibrated time period sufficient to accurately detect a catalyst""s conversion efficiency depends on the catalyst, engine and overall vehicle design. Specific calibrations for both the time duration and magnitude of fuel cycling for causing a catalyst temperature change are required due to large variations in engine characteristics such as displacement, cylinder number, manifold characteristics and other design parameters.